The field of the invention is in metal-gas and metal-metal secondary electrochemical energy storage cells and more particularly in the arrangement of the elements of such cells.
In many secondary electrochemical energy storage cells oxygen is evolved from the cathode during overcharge and to a lesser extent, during charge of the cells. Examples of such cells are nickel-hydrogen cells, nickel-cadmium cells, silver-hydrogen cells, silver-zinc cells and silver-cadmium cells.
The evolution of oxygen can produce undesirable effects within the cells and can impose undesirable constraints on the design and operation of the cells. Since no effective method has been found to completely eliminate the evolution of oxygen, various means of recombining it with minimal disruption of cell design and operation have been sought. These means typically involve transport of the oxygen from the cathode to a place where it can be chemically recombined.
Examples of undesirable effects of oxygen evolution are given here for nickel-hydrogen cells as an example of the class of metal-gas cells, and for nickel-cadmium cells as an example of the class of metal-metal cells, where the terms metal-gas and metal-metal refer to the nature of the cathode-anode couples of the cell. Similar effects apply to other members of these classes.
Examples of undesirable effects of oxygen evolution in nickel-hydrogen cells include: (1) melting of separators made of nylon or polypropylene as a result of locally excessive heat released when evolved oxygen gas passes through the separator and recombines with hydrogen at the anode; (2) expulsion or entrainment of electrolyte from the cathode as a result of oxygen gas bubbles moving through or out of the cathode; (3) maldistribution of water in the electrolyte as a result of local imbalances in the hydrogen-oxygen recombination rates; and (4) maldistribution of heat production as a result of local imbalances in the hydrogen-oxygen recombination rates.
An example of an undesirable effect of oxygen evolution in nickel-cadmium cells is the excessive cell pressure rise which results if the oxygen cannot be chemically recombined at an adequate rate.
Accommodation of these undesirable effects imposes undesirable constraints on the design and operation of the cells. For example, nickel-hydrogen cells should not use nylon or polypropylene separators in applications where oxygen evolution rates are sufficient to cause melting of those separators. Nickel-cadmium cells must be carefully controlled to prevent overcharge for very long at rates which will cause oxygen to be evolved faster than it can be recombined.
Typical structures in the prior art that are of interest are disclosed in U.S. Pat. No. 3,669,744 to patentees B. I. Tsenter et al, U.S. Pat. No. 3,826,684 to patentee S. F. Pensabene, U.S. Pat. No. 3,850,694 to patentees J. D. Dunlop et al, and U.S. Pat. No. 3,867,199 to J. D. Dunlop et al.